Fix UniToMultiPublisher race condition

[jponge]

• Protect UniToMultiPublisher from re-subscriptions when concurrent requests happen
• Refactor concurrency management in UniToMultiPublisher

Fixes #689

[codecov]

Codecov Report

Merging #691 (3091418) into main (5a631a2) will increase coverage by 0.06%.
The diff coverage is 76.47%.

❗ Current head 3091418 differs from pull request most recent head 6550edd. Consider uploading reports for the commit 6550edd to get more accurate results

@@             Coverage Diff              @@
##               main     #691      +/-   ##
============================================
+ Coverage     89.91%   89.98%   +0.06%     
- Complexity     2978     2980       +2     
============================================
  Files           374      374              
  Lines         11764    11770       +6     
  Branches       1476     1478       +2     
============================================
+ Hits          10578    10591      +13     
+ Misses          608      604       -4     
+ Partials        578      575       -3     

Impacted Files	Coverage Δ
...rye/mutiny/converters/uni/UniToMultiPublisher.java	86.84% <75.00%> (-0.66%)	⬇️
...subscription/SwitchableSubscriptionSubscriber.java	97.54% <100.00%> (+2.45%)	⬆️
...smallrye/mutiny/helpers/test/AssertSubscriber.java	90.22% <0.00%> (-1.51%)	⬇️
...perators/multi/processors/SerializedProcessor.java	87.12% <0.00%> (-1.00%)	⬇️
...operators/multi/builders/CollectionBasedMulti.java	93.54% <0.00%> (ø)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	85.04% <0.00%> (+0.31%)	⬆️
...allrye/mutiny/operators/uni/UniAndCombination.java	89.53% <0.00%> (+1.16%)	⬆️
...erators/multi/builders/SerializedMultiEmitter.java	82.27% <0.00%> (+1.26%)	⬆️
...mallrye/mutiny/operators/multi/FlatMapManager.java	98.61% <0.00%> (+2.77%)	⬆️
... and 1 more